Method for introduction of filling materials in liquid form into porous bodies

ABSTRACT

A product made by the method comprising forming a porous body of a first material, forming a plurality of recesses in a surface of such body, inserting wicks into such recesses and infusing a molten second material into the interior of the first material by injecting the second material into and through such wicks.

This invention relates to a method for introduction of filling materialsin liquid form into porous bodies. In particular, it relates to a methodin which the liquid filling materials after introduction into the porousbody react with at least one of its components, with the formation of atleast one chemical compound.

Often in the production of ceramic materials first a porous body isproduced by a thermal process step, such as, for example, sintering orpyrolysis, and is then compacted. This compaction can take place bypressing, that is, without adding another substance, or by one or moreinfiltration or impregnation steps, while the infiltration agent canremain unchanged and at least partially fills only the (open oraccessible) pores, or the infiltration agent reacts at least in one partwith at least one of the components of the porous body, with theformation of at least one chemical compound.

A field in which this method has acquired technical importance is theproduction of ceramic material based on silicon carbide, first a porouscarbon body being produced which then, by a reaction with liquid siliconor a silicon-containing melt with for the most part preservation of thegeometrical shape and dimensions (so-called “near net shape method”) ofthe carbon body, is converted into a body which largely consists of asilicon carbide which has been formed by the reaction of carbon andsilicon. At the conventionally used temperatures above the melting pointof silicon up to 1700° C., the beta-modification of the silicon carbidewhich is stable at these temperatures is formed.

There are various ways of introducing liquid silicon into porous,carbon-containing bodies. U.S. Pat. No. 5,432,253 discloses placing theporous body which is to be impregnated jointly with silicon in lump formon a carbon cloth (fabric or felt) and heating everything together tothe infiltration temperature, that is, a temperature at which thesilicon is molten liquid and has a low enough viscosity to penetrate byway of the cloth into the porous body and to be distributed in it.

Another method is known from U.S. Pat. No. 4,626,516 in which a moldwith infiltration holes and a reservoir with elementary silicon is used;a porous body is placed in the mold, and the holes are provided withcarbon wicks, the wicks being in contact with the porous body and thereservoir. At the operating temperature above the melting point ofsilicon, the silicon migrates through the wicks into the porous body.

U.S. Pat. No. 4,737,328 discloses covering a porous body with a powdermixture of silicon and boron nitride, and heating the arrangement abovethe melting point of the silicon, its penetrating into the body, and theboron nitride powder then being brushed off. Another method is coveringa porous carbon body with a moldable and hardenable mass of siliconpowder and a binder according to patent application WO1982/04248 orpatent application EP 0 995 730 A1. The advantage there is supposed tobe that the silicon is delivered exactly in the required amount.Finally, patent application DE 102 12 043 discloses placing the articlewhich is to be infiltrated with silicon on wicks of carbonized wood,these wicks with their lower end dipping into a metal melt.

The problem which is common to all these methods is that the siliconmelt is always supplied from the surface of the body to be infiltrated.Here, in the vicinity of the entry point there is always a great excessof silicon, while much less silicon is transported into the regionsfarther away. For two-dimensional delivery as in applications WO1982/04248 or EP 0 995 730 A1, liquid silicon is supplied over theentire covered surface, but a gradient forms over the thickness of thebody to be infiltrated. These gradients lead to inhomogeneity in theinfiltrated and potentially reacted bodies which occasions anintolerable imbalance, especially for parts turning at high speed, suchas brake disks or clutch disks.

Therefore the object of this invention is to devise a method which leadsto symmetrical and largely homogenous distribution of the infiltrationagent without the method thereby being complicated in an unacceptablemanner.

This object is achieved by supplying the infiltration agent in theinterior of the porous body.

Therefore the invention relates to a method for introduction of fillingmaterials into porous bodies in which the filling materials in liquidform are brought into contact with the porous bodies and are distributedin this porous body from at least one site within the porous body. Thereit is preferred that the filling materials react with at least onecomponent of the porous body with the formation of a chemical compound.Preferably the filling materials are transported into the interior ofthe porous body by way of at least one wick, and diffuse into the porousbody from the end of a wick located in the porous body.

Especially good results are obtained when the filling materials inliquid form are introduced at more than one site within the porous body,the introduction sites preferably being arranged symmetrically. Theintroduction sites are preferably made as blind holes, in the case offlush-fitting wicks a wick being inserted into a blind hole or in thecase of little play between the wick and blind hole the wick beingattached in the blind hole by suitable means, for example, by cementing.

Preferably the porous body contains carbon in the form of fibers orfiber bundles or textiles and additional carbon in the form of a porousmatrix. The liquid filling material there reacts with the carbon of theporous body at least partially to form carbides. To form ceramic partswith a matrix which contains silicon carbide, the infiltration agent isa silicon-containing melt, the porous body containing carbon. In theprocess silicon carbide is formed by the reaction of thesilicon-containing infiltration agent with the carbon of the porousbody. In addition to silicon, other elements such as iron, cobalt,nickel, titanium, boron, aluminum, chromium, manganese, molybdenum, andtungsten can also be contained in the infiltration agent, preferablythose which form a homogenous melt with silicon.

In the case of a disk-shaped or ring disk-shaped porous body it isespecially preferred that blind holes with a staggered diameter bedrilled in these bodies, these blind holes preferably being locatedcentrosymmetrically relative to their location around the axis ofrotation of the porous body.

The invention is explained in the following drawings.

FIG. 1 shows a top view of the lower boundary plane of a ringdisk-shaped body for a brake disk with perforation drilled holes,

FIG. 2 shows a cutaway view of FIG. 1 in which the staggered shape ofthe blind hole can be recognized,

FIG. 3 shows a section through one part of a ring disk-shaped body witha blind hole and an inserted wick.

The photographs show in

FIG. 4 a section through a siliconized ring disk, an insufficient amountof silicon having infiltrated from the undersurface by placement onporous wicks, and infiltration having been afterwards interrupted, andthe partially siliconized body being sawn off and photographed, and

FIG. 5 a section through a geometrically identical ring disk which hasbeen infiltrated according to the method according to the invention, aninsufficient amount of silicon having infiltrated from a blind hole withthe porous wick inserted, and infiltration having been interruptedafterwards, and the partially siliconized body being sawn off andphotographed.

FIG. 1 shows a top view of a ring disk 1 from the side of the ring disk1 provided with blind holes 3, in this embodiment live blind holes beinglocated near the inner periphery of the ring disk 1 and each offset by72°. Perforation drilled holes 5 are distributed over the surface of thering disk.

FIG. 2 shows a cutaway view of this top view, a blind hole being shownenlarged. Both the drilled hole 31 with the larger diameter and also thedrilled hole 32 with the smaller diameter can be recognized fromoverhead. The perforation drilled holes through the ring disk aredesignated as 5.

FIG. 3 shows a section through a ring disk 1 along the plane which isspanned by the axis of rotation of the ring disk 1 and the segmentIII-III. In the right part of the figure there is a blind hole 3 intowhich the wick 4 is inserted. As a result of the orientation of thepores, which in the case of wicks of carbonized wood are preferablyaligned in the longitudinal axis oldie wicks 4 according to the selectedpreferred orientation in the production of the wicks 4, and which in thecase of wicks 4 containing bundles of carbon fibers according to theabove explained preferred orientation of the fiber bundles are likewisearranged preferably parallel to the wick axis, the preferred entryregion of the liquid silicon into the ring disk to be infiltrated isformed by the step 33 in the blind hole 3 in the transition from thedrilled hole 31 with the larger diameter to the drilled hole 32 with thesmaller diameter of the blind hole 3.

Preferably the blind holes 3 are approx. 1.5 mm to approx. 4 mm,especially preferably 1.8 mm to 3 mm, from the surface of the ring diskopposite the drilled hole; its diameter in the region of the drilledhole 32 with the smaller diameter is preferably 2 mm to 4.5 mm andespecially preferably 2.5 mm to 4 mm. In the region of the drilledsurface the diameter of the drilled hole 31 is preferably at least twicethe diameter in the narrower part; favorable values are from 4 mm to 10mm. The depth of the region of the drilled hole 31 of the blind holewith the larger diameter here is preferably at least 3 mm and preferablynot more than 10 mm or 25% of the thickness of the component at thissite. The preferred length of the region of the drilled hole 32 with thesmaller diameter can be found by calculation from the indicatedpreferred regions for the depth of the blind hole 3 and the depth of theregion of the drilled hole 31.

Appropriate preferably cylindrical wicks 4, preferably of porous carbon,are inserted into the region of the drilled hole 31 of the blind holeswith a larger diameter with little play, preferable from 0.05 mm to 0.2mm. With little play the insertion of the wicks into the drilled hole 31is sufficient, in the case of greater play, attachment, for example, bycementing is preferred. Wicks of carbonized wood or porous carbonreinforced with carbon fibers are especially preferred, especially thosewicks being preferred which contain roving bundles placed parallel incylindrical shape, which bundles are bound by carbonizable binders suchas phenolic resins or pitches or their mixture, before use these bindershaving been carbonized by heating with the exclusion of oxygen totemperatures from approx. 750° C. to 1300° C.

The infiltration agent which is diffusing through the wick 4, preferablysilicon or a melt containing it, is distributed in the region of thedrilled hole 32 and fills it when the diameters as stated above areobserved in this hole; from this reservoir the infiltration agent candiffuse perpendicular to the axis of rotation of the ring disk into theinterior of the ring disk and for the preferred orientation ofreinforcing fibers in the porous carbon body which preferably forms thering disk in the plane of the ring disk, that is, perpendicular to theaxis of rotation, causes uniform advance of the front of theinfiltration agent out of the region of the drilled hole 32.

The advantage of uniform infiltration achieved by the invention is shownby the photographs of FIGS. 4 and 5, in FIG. 4 the supply of liquidsilicon at 1500° C. and a pressure of 5 hPa having taken place from thebottom of the ring disk in the ground pick by wicks which are not shownhere and which have been removed from the ring disk by partialsiliconization; the wick locations are marked by arrows. The silicon canbe recognized in the photographs in the form of white spots. It can bediscerned that diffusion in the direction of the front, that is, fromthe bottom in the figure to the top, takes place very nonuniformly;diffusion in the horizontal direction is also more rapid than in thevertical direction. The orientation of the reinforcing fibers ishorizontal, as can be recognized in the figure; this orientation is dueto the required strength of the ring disk.

FIG. 5 shows a sawn ring disk in which the supply of the infiltrationagent silicon has taken place from the zone of the drilled hole 32viewed in the horizontal direction (direction of the arrow) in thefigure, the uniform flow front of the silicon in the ring disk providedwith reinforcing fibers in the horizontal direction and the siliconcarbide imaged gray in the photographs being clearly recognizable. Thesame amount of silicon (determined by weighing) under otherwiseidentical conditions was infiltrated into the ring disk with the samegeometry as in the example shown in FIG. 4, with infiltration from theundersurface of the ring disk. Infiltration takes place more quickly andmore uniformly than in the example shown in FIG. 4.

The invention claimed is:
 1. A product made by the method comprising:forming a porous body of a first material; forming a plurality ofrecesses each with an enlarged portion in a surface of said body;inserting wicks into the enlarged portions of said recesses; andinfusing a molten second material into the interior of said firstmaterial by injecting said second material into and through said wicks,providing an in depth, homogeneous distribution of said second materialin said first material.
 2. The product according to claim 1 wherein saidfirst material includes carbon and said infused second material includesmolten silicon which chemically reacts with said carbon to form acarbide.
 3. The product according to claim 2 wherein said first materialincludes a fibrous material.
 4. The product according to claim 3 whereinsaid fibrous material is embedded in a porous matrix of carbon.
 5. Theproduct according to claim 1 wherein said body is annularly configuredand said recesses are dispersed on a face of said surface.
 6. Theproduct according to claim 1 wherein said recesses extend through amajor portion of a thickness of said body.
 7. The product according toclaim 1 wherein said enlarged portion of said recess extends 25% of thedepth of said recess.
 8. The product according to claim 1 wherein adiameter of each of said enlarged portions is twice the diameter of eachof said recesses.
 9. The product of claim 1 wherein the diameter of eachof said recesses is in the range of 2 mm to 4.5 mm and the diameter ofeach of said enlarged portions of said recesses is in the range of 3 mmto 10 mm.
 10. The product of claim 1 wherein each of said wickscomprises one of carbonized wood and porous carbon reinforced withcarbon fibers.
 11. The product of claim 1 wherein said body comprises adisk of a brake assembly.